An electronic device such as a laser printer has a paper feed roller (i.e., driven unit) coupled with a motor and disposed inside a main unit casing, and this motor drives the paper feed roller to rotate and deliver paper to a predetermined section.
The above motor comprises a stator having a plurality of magnetic poles disposed at first predetermined intervals in a circumferential direction along an outer periphery thereof, and a rotor disposed around the outer periphery of the stator. The rotor has a structure comprising magnets disposed at second predetermined intervals in a circumferential direction along an inner periphery thereof with their poles magnetized alternately to have different polarities.
The stator is also provided with an extended portion formed on each of the magnetic poles in a manner to extend from the base of the magnetic pole into a direction generally parallel to the magnets to thereby improve a driving efficiency.
That is, the magnets are made larger in width (i.e., the width in a direction orthogonal to the circumferential direction) than a width at the base of the magnetic poles of the stator in the same direction (i.e., the direction orthogonal to the circumferential direction) in order to bring the magnets as close as possible to a magnetic sensing element for magnetically sensing rotation of the rotor. It is for this reason that the conventional motor comprises the extended portions extended from the base of the magnetic poles into the direction generally parallel to the magnets to gain a high driving efficiency. Patent literature 1, for instance, discloses a technique similar to the above.
In the conventional motor, the extended portions formed to extend from the base of the magnetic poles into the direction parallel to the magnets in the manner as described above can increase an area where the magnets of the rotor confront the magnetic poles of the stator. It has been believed in general that the driving force can be increased by increasing the confronting area, and hence the driving efficiency as well.
According to the studies made by the inventors of this application, however, it was found not necessarily possible to increase the driving force by simply providing the extended portions.
In other words, it is the area where the magnets of the rotor confront the magnetic poles of the stator, enlargement of which can improve the driving force according to the general conception. This suggests that the extended portions from the electrodes of the stator be increased as large as possible. When the extended portions are enlarged an amount of magnetic flux from the confronting magnets also increases proportionally however, and this gives rise to a problem that a magnetic saturation can result in a magnetic path of the stator communicating the magnetic poles. There is also another problem that the extended portions become a factor of reducing the driving efficiency due to a large eddy current loss generated therein when the extended portions have a large thickness since they receive the magnetic flux of a considerable amount in a direction orthogonal thereto from the magnets.    Patent Literature 1: Japanese Patent Unexamined Publication, No. 1997-285044